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KCC' Quizzes AQQ270 about a difference amplifier challenge

1. Quote of the week: 

2. New challenge AQQ270 about a Difference Amplifier challenge (this is a proposal from our colleague

Martin Walker, Product Marketing Leader at ADI UK – warm thanks to him!):

Below is the classic difference amplifier circuit.

Questions:

 1. What is the Common Mode Rejection Ratio (CMRR) if we have perfectly matched resistors?  [Assume the amplifier’s CMRR is 120dB]

 2. What is the CMRR if the resistors have a tolerance of 0.1%?

 3. What is the CMRR if the resistors have a tolerance of 1%?

 4. What input resistance does V2 ‘see’?

 5. What input resistance does V1 ‘see’?

 6. If the difference amplifier was AC-coupled, what impact would this inequality in input impedance have?

Good luck!



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[edited by: GenevaCooper at 1:18 PM (GMT -4) on 20 Sep 2024]
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  • In theory, with perfect resistors, you are correct, the CMRR will just be the CMRR of the amplifier.

    One of my favorite sayings is, in theory there is no difference between theory and practice but in practice there usually is...

    This quiz was actually inspired by a real customer question where they were seeing a DC offset on their motor current measurement that increased linearly with the common-mode voltage.  They thought the amplifier might be faulty but in reality they were just seeing the effect of resistor tolerance on CMRR.

    The CMRR is actually significantly worse than the numbers you have given.  gpiarino used an estimate and vanderghast used a simulation to get close to the real figures and there is a better approximation given in one of our Instrumentation Amplifier app notes...

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  • In theory, with perfect resistors, you are correct, the CMRR will just be the CMRR of the amplifier.

    One of my favorite sayings is, in theory there is no difference between theory and practice but in practice there usually is...

    This quiz was actually inspired by a real customer question where they were seeing a DC offset on their motor current measurement that increased linearly with the common-mode voltage.  They thought the amplifier might be faulty but in reality they were just seeing the effect of resistor tolerance on CMRR.

    The CMRR is actually significantly worse than the numbers you have given.  gpiarino used an estimate and vanderghast used a simulation to get close to the real figures and there is a better approximation given in one of our Instrumentation Amplifier app notes...

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